This invention relates to a permanent magnet type stepping motor, and more particularly to a permanent magnet type stepping motor adapted to exhibit increased rotational angle accuracy and static angle accuracy and minimize a reduction in static torque.
A typical two-phase permanent magnet type stepping motor which has been conventionally known in the art is generally constructed in such a manner as shown in FIGS. 4 and 5, wherein FIG. 4 is a development view of the stepping motor and FIG. 5 shows relationship between pole teeth of a stator and magnetic poles of a rotor.
In FIG. 4, reference numeral 1 designates a rotor including a cylindrical permanent magnet element 7 securely mounted on a cylindrical bushing 5 fixed on a revolving shaft 3. The revolving shaft 3 is supported by two bearings (not shown). The permanent magnet element 7 is constructed into a cylindrical configuration so as to permit n (n: a positive integer of 4 or more) N magnetic poles and n S magnetic poles to alternately appear at equal pitches thereon in a circumferential direction of the revolving shaft 3. The conventional stepping motor also includes a stator 9 arranged on an outer periphery of the rotor 1 while being mounted in a housing (not shown). The stator 9 includes a first claw pole type yoke unit 11 and a second claw pole type yoke unit 25. The first and second claw pole type yoke units 11 and 25 are arranged in juxtaposition to each other in an axial direction of the revolving shaft 3. The first and second claw pole type yoke units 11 and 25 are shifted from each other in the circumferential direction by a distance one quarter (xc2xc) as large as predetermined pitches P between the magnetic poles of the permanent magnet element 7. The first claw pole type yoke unit 11 includes a first yoke 13 and a second yoke 17 respectively including n pole teeth .15 and 19 arranged opposite to the permanent magnet element 7 at a predetermined interval in a radial direction of the revolving shaft 3 and positioned at predetermined pitches P in the circumferential direction. The first claw pole type yoke unit 11 also includes an exciting winding 21 constructed so as to permit the n pole teeth 15 of the first yoke 13 and the n pole teeth 19 of the second yoke 17 to be excited at polarities different from each other, respectively. The exciting winding 21 is wound on a bobbin 23.
The second claw pole yoke unit 25 likewise includes a first yoke 27 and a second yoke 31 respectively including n pole teeth 29 and 33 arranged opposite to the permanent magnet element 7 at a predetermined interval in the radial direction of the revolving shaft 3 and positioned at predetermined pitches P in the circumferential direction. The second claw pole type yoke unit 25 also includes an exciting winding 35 constructed so as to permit the n pole teeth 29 of the first yoke 27 and the n pole teeth 33 of the second yoke 31 to be excited at polarities different from each other, respectively. The exciting winding 35 is likewise wound on a bobbin 37.
In the conventional permanent magnet type stepping motor thus constructed, as shown in FIG. 5, the pitch P between each adjacent two of the pole teeth 15, that between each adjacent two of the pole teeth 19, that between each adjacent two of the pole teeth 29, that between each adjacent two of the pole teeth 33, and that between two magnetic poles of the permanent magnet element 7 are typically set to be identical with each other or to have the same angle.
Unfortunately, the conventional permanent magnet type stepping motor thus constructed is deteriorated in rotational angle accuracy and static angle accuracy as compared with a hybrid type stepping motor. In order to eliminate such a problem, a variety of techniques were proposed as disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 245929/1995, Japanese Patent Application Laid-Open Publication No. 245930/1995, Japanese Patent Application laid-Open Publication No. 127024/1998, Japanese Patent Application Laid-Open Publication No. 248232/1998 and the like. The techniques proposed are generally constructed so as to vary a configuration of pole teeth or shift one of two yokes from the other yoke at a predetermined pitch in a circumferential direction of a revolving shaft, to thereby reduce a detent torque, resulting in eliminating the above-described problem. Alternatively, in the proposed techniques, it is employed that a correction pole magnet of a phase opposite to a high frequency contained in electromotive force induced due to relative movement between a rotor and a stator is arranged on a magnetic pole of the rotor to suppress a high-frequency component, to thereby eliminate the problem. In the prior art, pitches between pole teeth of each of yokes of claw pole type yoke units are rendered constant.
As described above, those skilled in the art attempted to solve the above-described problem while rendering the pitches between the pole teeth of each of the yokes of the claw pole type yoke units constant. However, this fails to increase rotational angle accuracy and static angle accuracy. Also, the conventional rotor having the correction magnetic poles provided thereon causes a problem of substantially reducing a composite or synthesized static torque.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a permanent magnet type stepping motor which is capable of exhibiting both increased rotational angle accuracy and static angle accuracy and minimizing a reduction in static torque.
In accordance with the present invention, a permanent magnet type stepping motor is provided. The permanent magnet type stepping motor includes a rotor having a permanent magnet element fixed on a revolving shaft and a stator including two or more claw pole type yoke units juxtaposed to each other in an axial direction thereof. The permanent magnet element is constructed into a cylindrical configuration so as to permit n (n: a positive integer of 4 or more) N magnetic poles and n S magnetic poles to alternately appear at equal pitches thereon in a circumferential direction of the revolving shaft. The claw pole type yoke units each include first and second yokes and an exciting winding. The first and second yokes each include n pole teeth arranged opposite to the permanent magnet element at a predetermined interval in a radial direction of the revolving shaft and positioned at predetermined pitches in the circumferential direction. The exciting winding is constructed so as to permit the n pole teeth of the first yoke and the n pole teeth of the second yoke to be excited at polarities different from each other. The first and second yokes are combined with each other so that the n pole teeth of the first yoke and the n pole teeth of the second yoke are engaged with each other while being kept from being contacted with each other or are alternated with each other while being kept uncontacted with each other.
In the present invention, the respective adjacent two pole teeth of the claw pole yoke unit are arranged at constant pitches. Supposing that a magnetic pole center-magnetic pole center length between each adjacent two magnetic poles of the n N magnetic poles or n S magnetic poles of the rotor is defined to be one pitch (xe2x80x9cpitch between centers of homopolar magnetic polesxe2x80x9d, xe2x80x9cmagnetic pole center-magnetic pole center pitch between homopolar magnetic polesxe2x80x9d or xe2x80x9chomopolar magnetic pole center-magnetic pole center pitchxe2x80x9d) or a sum of magnetic pole widths of the N and S magnetic poles adjacent to each other is defined to be one pitch (xe2x80x9cheteropolar magnetic pole width pitchxe2x80x9d), the pitches are not defined to be constant at 360xc2x0/n but defined to be two or more pitches different from each other which are within a range of between more than 270xc2x0/n and less than 450xc2x0/n. It is a matter of course that a sum of 2n pitches between the respective adjacent two magnetic poles of the 2n magnetic poles is 360xc2x0. Also, magnetic center-magnetic center pitches between the respective adjacent two magnetic poles of the rotor are not defined to be constant at 180xc2x0/n but defined to be two or more pitches different from each other which are between more than 135xc2x0/n and less than 225xc2x0/n.
Basically, the present invention is featured in that the above-described homopolar magnetic pole center-magnetic pole center pitch or heteropolar magnetic pole width pitch and the magnetic center-magnetic center pitch between the adjacent two magnetic poles of the rotor are varied within the above-described angular range while keeping the pitch between the pole teeth of the yokes of the claw pole type yoke unit constant. When the pitch between the pole teeth of each of the yokes of the claw pole type yoke unit is rendered constant as in the prior art, a phase of a partial static torque partially occurring between the pitches or a phase from a viewpoint of an electrical angle is caused to be identical. In the prior art, it is attempted to vary a configuration of the pole teeth to vary characteristics of the partial static torque and a configuration thereof, resulting in approaching a synthesized static torque which is obtained due to synthesis of a plurality of partial static torques to a sinusoidal wave. On the contrary, in the present invention, the pitch between the pole teeth of the two yokes adjacent to each other is rendered constant, whereas the magnetic center-magnetic center pitch between adjacent two magnetic poles of the rotor and the homopolar magnetic pole center-magnetic pole center pitch or heteropolar magnetic pole width pitch is shifted to shift a phase of each of the partial static torques, to thereby approach characteristics of the synthesized static torque to a sinusoidal wave.
Basically, so long as such construction permits characteristics of the synthesized static torque to be approached to a sinusoidal wave or attains a reduction in detent torque and an increase in rotational angle accuracy and static angle accuracy as compared with the case that the pitch between the magnetic poles of the rotor is constant, a manner of arrangement of the 2n pitches, n homopolar magnetic pole center-magnetic pole center pitch or heteropolar magnetic pole width pitch or a pitch pattern thereof may be varied as desired. Although an arrangement manner or a pitch pattern which leads to the best results has not been found yet, the inventors have found that a variation in pitches within the above-described conditions or ranges permits a specific harmonic component to be reduced or removed, leading to an improvement in characteristics of the synthesized static torque as compared with the prior art. The construction of the present invention that the pitches between the respective adjacent two magnetic poles of the rotor are rendered nonconstant permits a variety of pitch patterns to be employed unless magnetic balance of the stator is highly deteriorated or so long as the magnetic balance is somewhat ensured.
Now, preferred pitch patterns of the rotor which have been found up to date will be exemplified.
In a first pitch pattern, the rotor includes homopolar magnetic poles of n/m (m: a divisor of n other than 1 and n) in each of angular ranges (mechanical angles) of 360xc2x0/n. Also, in the first pitch pattern, a magnetic center-magnetic center pitch between each adjacent two of the n/m homopolar magnetic poles positioned in each of the angular ranges and a magnetic center-magnetic center pitch between each two homopolar poles positioned on each of both sides of adjacent two of the angular ranges are rendered different from each other in order to reduce a detent torque to increase rotational angle accuracy and static angle accuracy and minimize a reduction in static torque. In order to facilitate design of the permanent magnet type stepping motor and manufacturing thereof when the first pitch pattern is employed, the present invention may be constructed in such a manner that the magnetic center-magnetic center pitch between adjacent two of the n/m homopolar magnetic poles positioned in each of the angular ranges is constant and the magnetic center-magnetic center pitch between each two homopolar poles positioned on each of both sides of adjacent two of the angular ranges is constant.
In a second pitch pattern, the rotor includes n magnetic pole pairs each constituted by two heteropolar magnetic poles adjacent to each other. Also, in the second pitch pattern, respective two sets of magnetic pole pairs which are arranged adjacent to each other at every third interval have heteropolar magnetic pole width pitches defined to be different from those of respective two sets of the remaining magnetic pole pairs arranged adjacent to each other at every third interval.
A variation in pitch between the magnetic poles of the rotor when the above-described first pitch pattern is employed may be carried out using any one of two magnetic pole arrangement manners. In a first magnetic pole arrangement manner, the pitch a is reduced as compared with an equal arrangement pitch (360xc2x0/n). This is referred to as xe2x80x9cshort pitch arrangement mannerxe2x80x9d. In the short pitch arrangement manner, the magnetic center-magnetic center pitch a between each two of the n/m homopolar magnetic poles positioned in each of the angular ranges is defined to be within a range of (90xc2x0/n)[4xe2x88x92m/(nxe2x88x92m)] less than a less than (360xc2x0/n). Also, the magnetic center-magnetic center pitch b between each adjacent two of the magnetic poles positioned on each of both sides of each adjacent two of the angular ranges is defined to be within a range of (360xc2x0/n) less than b less than (450xc2x0/n). In the first arrangement manner, the pitch a is reduced as compared with the conventional pitch 360xc2x0/n and the pitch b is larger than the pitch 360xc2x0/n.
In a second magnetic pole arrangement manner, the pitch a is set to be larger than the equal arrangement pitch 360xc2x0/n. This is referred to as xe2x80x9clong pitch arrangement mannerxe2x80x9d. In the long pitch arrangement manner, the magnetic center-magnetic center pitch a between each adjacent two of the n/m homopolar magnetic poles positioned in each of the angular ranges is defined to be within a range of (45xc2x0/n)[4+m/(nxe2x88x92m)] greater than a greater than (180xc2x0/n) and the magnetic center-magnetic center pitch b between each adjacent two of the homopolar magnetic poles positioned on each of both sides of a boundary between each adjacent two of the angular ranges is defined to be within a range of (180xc2x0/n) greater than b greater than (90xc2x0/n).
In the second magnetic pole arrangement manner, the pitch a is increased as compared with the conventional pitch 180xc2x0/n and the pitch b is smaller than the pitch 360xc2x0/n.
The first and second magnetic pole arrangement manners described above each permit partial static torques occurring at the pitches to be successively shifted to either a negative side or a positive side within an electrical angle of 90xc2x0. This results in a detent torque or a harmonic component being decreased, leading to an increase in rotational angle accuracy and static angle accuracy.
When a magnetic center is not varied in each of the angular ranges, the pitches a in each of the angular ranges may be varied within the above-described conditions and the pitches b of m in number may be varied within the above-described conditions. Nevertheless, in order to facilitate design of the permanent magnet type stepping motor, the pitches a and b are preferably constant. In this instance, values of the pitches a and b are set so as to meet relationship (nxe2x88x92m)xc3x97a+mxc3x97b=360xc2x0. This not only facilitates design of the permanent magnet type stepping motor and manufacturing thereof but permits rotational angle accuracy and static angle accuracy to be increased.